Spinal cord stimulation with interferential current

ABSTRACT

A stimulator and a method for the treatment of intractable pain syndromes by electrical stimulation of the spinal cord is disclosed in which implantable electrodes positioned around a targeted area of the spinal cord transmit an interferential current that has a base medium frequency alternating current between 500 Hz-20 KHz. A digital signal processor generates a sine-wave-like waveform from a pulse generator which after further processing is used to generate at least two circuits for use in producing the interferential current. An effective area of stimulation is controlled by the quantity of electrodes, positioning of the electrodes and electrode cross pattern orientation. Amplitude modulation of electrical circuits created at the electrode placements also augments the effective area of stimulation. The stimulator and method reduce accommodation of the body to the electrical stimulation and provide deeper penetration of the resultant signal.

REFERENCE TO RELATED APPLICATION

[0001] The present application claims the benefit of U.S. ProvisionalPatent Application No. 60/441,326 filed Jan. 22, 2003 whose disclosureis hereby incorporated by reference in its entirety into the presentdisclosure.

FIELD OF THE INVENTION

[0002] The present invention is generally related to spinal cordstimulation and, more particularly, is related to an apparatus andmethod for the electrical stimulation of the spinal cord using aninterferential current pattern for treating chronic pain conditions.

BACKGROUND OF THE INVENTION

[0003] Electrical stimulation of the posterior spinal cord, spinal cordstimulation (SCS), has developed into an effective therapeutic tool fortreating chronic pain conditions. However, very little is known aboutthe sites of activation or the neural mechanisms evoked by SCS thatrelieve pain and promote changes in the function of somatic and visceralstructures.

[0004] Spinal Cord Stimulation is most commonly used for patients withchronic intractable pain syndromes. It has also been useful for treatingmovement disorders and is occasionally used following head injuries.However, one complication with SCS is that of accommodation orhabituation to the stimulation signal. Companies that manufacture spinalstimulation devices have developed complex stimulation programs anddevoted chapters on techniques to reduce the problem of accommodationduring SCS (Alfano S, Darwin J, Picullel B: Spinal Cord Stimulation,Patient Management Guidelines for Clinicians, Medtronic, Inc.).Accommodation is when the body habituates or becomes accustomed to anactivity or signal and then starts to ignore or ‘tune it out’. Byvarying the signal or keeping the focal point of the signal moving,accommodation can be minimized. The concept of using interferentialstimulation with implantable leads to decrease the problem ofaccommodation might prove to be advantageous.

[0005] Dorsal Column Stimulation (DCS) or SCS using an electricalinterferential current pattern has shown to be a cost benefit intreating chronic pain disorders in patients (Dorsal column stimulation:cost to benefit analysis; Acta Neurochir Suppl (Wien), 52( ): 121-3,1991).

[0006] SCS stimulates the dorsal column in a somewhat superficial manneras pointed out by Holsheimer (Holsheimer J: Which Neuronal Elements areactivated Directly by Spinal Cord Stimulation, Neuromodulation, Volume5, Number 1: 25-31,2002). The electrodes are normally attached to thedura matter in the epidural space, and most of the current distributionremains in the cerebrospinal fluid (CSF) and does not project deeplyinto the dorsal column. Providing an interferential component to theelectrode array of the SCS allows the crossing of the two signalswherein the resultant additive effect of the beat frequency producesdeeper penetration of the signal and a higher resultant amplitude at thestimulation site. The interferential current would recruit largernumbers of dorsal column fibers and provide greater levels of painrelief and benefit to intractable pain patients.

[0007] Thus, a heretofore unaddressed need exists in the industry toaddress the aforementioned deficiencies and inadequacies with regard toaccommodation or habituation to the spinal cord stimulation signal whenused in the treatment of chronic pain syndromes.

SUMMARY OF THE INVENTION

[0008] Embodiments of the present invention provide an apparatus andmethod for the treatment of chronic pain syndromes using electricalstimulation of the spinal cord. The present invention utilizes aninterferential current that has a base medium frequency alternatingcurrent between 500 Hz and 20 KHz. An interferential current is set upbetween two circuits that are arranged in a cross-pattern on thesubject's targeted area of stimulation. Where the circuits superimposein a cross-pattern, the resultant beat frequency will be the differencebetween the frequencies of the two circuits and the amplitude will beadditive and greater than either circuit alone. The range of the beatfrequency is usually between 1-250 Hz. Multiple levels of stimulationcan be treated depending upon the electrode placement, pairing andmodulation pattern selected. The range of output would be from 0-11volts per circuit depending on the patient's needs and the pulse widthis commonly set at 210 microseconds but it could range from 10-600microseconds. The amplitude can be modulated in the respective circuitsto increase the area of targeted stimulation. This type of current(Interferential) provides improved directional control, decreasedaccommodation/habituation and increased depth of penetration incomparison to other standard implantable stimulation systems and theiraccompanying surgical leads. The amplitudes of the outputs in therespective circuits may be modulated to increase the area of targetedstimulation. Interferential current allows improved directional controland depth of penetration in comparison to other stimulation techniques.

[0009] Briefly described, in architecture, one embodiment of theinvention, among others, can be implemented as follows.

[0010] Digital signal processors (DSPs) are used for improving theaccuracy and reliability of digital signals that are used extensively inthe communications field. Digital signal processing works bystandardizing or clarifying the output of a digital signal. In thisembodiment, the digital signal processor is used to shape multiplepulsatile waveforms to approximate the output of a sine-wave generator.In another embodiment of the invention, the digital signal processor isreplaced with a field-programmable gate array (FPGA). An FPGA is anintegrated circuit that can be programmed in the field after it ismanufactured and therefore allows users to adjust the circuit output asthe needs change. Both the DSP and the FPGA process a digital signalinto a pseudo-sine-wave current waveform from the digital pulsesgenerated by a pulse generator. The pseudo-sine-wave current waveform istransmitted through implantable quadripolar leads with eight electrodesat a targeted area creating a pair of interferential currents.

[0011] Other systems, methods, features, and advantages of the presentinvention will be or become apparent to one with skill in the art uponexamination of the following drawings and detailed description. It isintended that all such additional systems, methods, features, andadvantages be included within this description, be within the scope ofthe present invention, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Many aspects of the invention can be better understood withreference to the following drawings. The components in the drawings arenot necessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present invention. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

[0013]FIG. 1 is a perspective view of an interferential current set upby two circuits that are arranged in a cross pattern;

[0014]FIG. 2 is a perspective view of an interferential current patternindicating the current intensity level and area of beat frequencyformation;

[0015]FIG. 3 is a perspective view illustrating the effective area ofstimulation resulting from the crossing of separate circuits; and

[0016]FIG. 4 is a diagram illustrating interferential stimulation usingtwo implantable quadripolar leads.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0017] A preferred embodiment of the invention and modifications thereofwill now be described with reference to the drawings.

[0018]FIG. 1 shows a stimulator 100 for the electrical stimulation ofthe spinal cord utilizing an interferential current 110 that has a basemedium frequency alternating current within the range of 500 Hz-20 KHz.The interferential current 110 is set up between two circuits 118, 120that are arranged in a cross-pattern. A first pair of implantableelectrodes 108, 208 are positioned on a subject's spinal column 112,preferably the dorsal column, at one set of diagonal corners of atargeted area 214 (see FIG. 2). A second pair of implantable electrodes108, 208 is then positioned at the other set of diagonal corners of thetargeted area 214. Preferably, the electrodes 108 are attached to thedura matter in the epidural space. A digital signal processor 102 isconnected to the first and second pairs of surface electrodes 108. Whena signal generating source 104 is connected to the digital signalprocessor 102, a sine-wave-like waveform signal output 106 is created.The digital signal processor 102 improves the accuracy and reliabilityof digital signals. The digital signal processor 102 processes themultiple pulses 116 from the signal generating source 104 to approximatea sine-wave (pseudo-sine-wave or sine-wave-like). Thus, that type ofcurrent recruits larger numbers of dorsal column fibers and providesgreater levels of pain relief.

[0019] The digital signal processor 102 generates individual pulses 106of differing widths and resultant amplitudes. Preferably, the pulsewidth is set at 210 microseconds, but can range from 50-600microseconds. When those differing pulses 106 are driven into atransformer (not shown), the pseudo-sine-wave is produced. A pulsegenerator 104 is connected to the digital signal processor 102 andsupplies a pulsed digital signal output 116 to the digital signalprocessor 102. The digital signal 106 processed by the digital signalprocessor 102 creates a first circuit 118 and a second circuit 120 atthe first and second pairs of surface electrodes 108, 208, respectively.Preferably, the range of output of the electrical circuits 118, 120 are0-11 volts per circuit, depending on the patient's needs for paintreatment. Where the first and second circuits 118, 120 superimpose(cross), the resultant beat frequency (which is preferably between 1 and250 beats/second) will be the difference between the frequencies of thetwo circuits, and the amplitude will be additive and greater than eithercircuit alone (FIG. 2).

[0020] Multiple target areas of the spinal cord can be treated dependingupon the quantity and placement of the first and second pairs ofelectrodes 308, and by modulating the amplitudes of the outputs of thefirst and second circuits 318, 320 (see FIG. 3). Modulating the outputsof the first and second circuits 318, 320 increases the area of thetargeted stimulation. The depth of modulation can vary from 0 to 100%and depends on the direction of the currents established by the firstand second circuits 318, 320. It has been shown that when the first andsecond circuits 318, 320 intersect at 90°, the maximum resultantamplitude and the deepest level of modulation is half-way between thetwo circuits (45° diagonally). (See FIG. 2). Hence, the target area ofstimulation can be augmented by modulation of the amplitudes of theoutputs of the two circuits.

[0021]FIG. 4 illustrates two interferential currents 406 withsine-wave-like waveforms that are produced by two implantablequadripolar leads 409. Each quadripolar lead 409 includes fourelectrodes 408 for a total of eight. The two quadripolar leads 409 allowa greater target treatment stimulation area of the spinal cord. However,the invention could also apply to the use of two bipolar or octapolarlead systems, and other suitable devices. The electrodes could beactivated in various combinations and patterns, and not just as shown inthe drawings.

[0022] A field-programmable gate array (not shown) can also be used toshape multiple pulsatile waveforms to approximate the output of asine-wave generator instead of the digital signal processor 102described above. The FPGA is an integrated circuit that can beprogrammed in the field after it is manufactured and allows its user toadjust the circuit output as desired. In an alternative embodiment, thedigital signal processor may be replaced with the FPGA. Whereas DSPprocessors typically have only eight dedicated multipliers at theirdisposal, a higher end FPGA device can offer up to 224 dedicatedmultipliers plus additional logic element-based multipliers as needed.That allows for complex digital signal processing applications such asfinite impulse response filters, forward error correction,modulation-demodulation, encryption and applications such as utilized inthe present invention.

[0023] It should be emphasized that the above-described embodiments ofthe present invention, particularly, any “preferred” embodiments, aremerely possible examples of implementations, merely set forth for aclear understanding on the principles of the invention. Many variationsand modifications may be made to the above-described embodiment(s) ofthe invention without departing substantially from the spirit andprinciples of the invention. All such modifications and variations areintended to be included herein within the scope of this disclosure andthe present invention and protected by the following claims.

We claim:
 1. An electrical stimulator for the treatment of intractablepain syndromes, comprising: an interferential current generator forgenerating an interferential alternating current output by using commonsine wave generators with a base medium frequency of at least 500 Hz butno more than 20 KHz; and at least two pairs of implantable electrodesconnected to said interferential current producing generator and locatedat predetermined locations on a subject's spinal cord wherein each pairof implantable electrodes produces a separate electrical circuit.
 2. Thestimulator of claim 1, wherein said interferential current generatorcomprises: a pulse generator that generates digital signal pulses; and adigital signal processor connected to said pulse generator thatprocesses the digital signal pulses to approximate a sine-wave-likeoutput waveform.
 3. The stimulator of claim 1, wherein saidinterferential current generator comprises: a pulse generator thatgenerates digital signal pulses; and a field-programmable gate arrayconnected to said pulse generator that processes the digital signalpulses to approximate a sine-wave-like output waveform.
 4. Thestimulator of claim 1, wherein said interferential current includes aresultant beat frequency of no more than 250 Hz.
 5. The stimulator ofclaim 1, wherein said interferential current includes a voltage outputof 11 volts maximum for each circuit.
 6. The stimulator of claim 1,wherein said interferential current includes a pulse width of 210microseconds.
 7. The stimulator of claim 1, wherein said interferentialcurrent includes a pulse width with a range of at least 10 microsecondsbut no more than 600 microseconds.
 8. The stimulator of claim 1, whereintwo quadripolar leads are used to produce two interferential currents.9. An electrical stimulator for the treatment of intractable painsyndromes, comprising: a pulse generator that generates digital signalpulses; a digital signal processor connected to said pulse generatorthat generates a sine-wave-like output waveform that is furtherprocessed into first and second circuits; and two pairs of implantableelectrodes connected to said digital signal processor and positioned ona subject's spinal cord at predetermined locations to produce aninterferential current output waveform from said first and secondcircuits.
 10. The stimulator of claim 9, wherein said interferentialcurrent output waveform includes a base medium frequency of at least 500Hz but no more than 20 KHz.
 11. The stimulator of claim 9, wherein saidinterferential current output waveform includes a resultant beatfrequency of no more than 250 Hz.
 12. An electrical stimulator for thetreatment of intractable pain syndromes, comprising: a pulse generatorthat generates digital signal pulses; a field-programmable gate arrayconnected to said pulse generator that generates a sine-wave-like outputwaveform that is further processed into first and second circuits; andtwo pairs of implantable electrodes connected to said field-programmablegate array and positioned on a subject's spinal cord at predeterminedlocations to produce an interferential current output waveform from saidfirst and second circuits.
 13. The stimulator of claim 12, wherein saidinterferential current output waveform includes a base medium frequencyof at least 500 Hz but no more than 20 KHz.
 14. The stimulator of claim12, wherein said interferential current waveform includes a resultantbeat frequency of no more than 250 Hz.
 15. An electrical stimulator forthe treatment of intractable pain syndromes, comprising: aninterferential current generator for generating an interferentialalternating current output with a base medium frequency of at least 500Hz but no more than 20 KHz; and at least two pairs of implantableelectrodes connected to said interferential current producing generatorand located at predetermined locations on a subject's dorsal columnwherein each pair of implantable electrodes produces a separateelectrical circuit.
 16. The stimulator of claim 15, wherein saidinterferential current generator comprises: a pulse generator thatgenerates digital signal pulses; and a digital signal processorconnected to said pulse generator that processes the digital signalpulses to approximate a sine-wave-like output waveform.
 17. Thestimulator of claim 15, wherein said interferential current generatorcomprises: a pulse generator that generates digital signal pulses; and afield-programmable gate array connected to said pulse generator thatprocesses the digital signal pulses to approximate a sine-wave-likeoutput waveform.
 18. The stimulator of claim 15, wherein saidinterferential current includes a resultant beat frequency of no morethan 250 Hz.
 19. The stimulator of claim 15, wherein said interferentialcurrent includes a voltage output of 11 volts maximum for each circuit.20. The stimulator of claim 15, wherein said interferential currentincludes a pulse width of 210 microseconds.
 21. The stimulator of claim15, wherein said interferential current includes a pulse width with arange of at least 10 microseconds but no more than 600 microseconds. 22.The stimulator of claim 15, wherein two quadripolar leads are used toproduce two interferential currents.
 23. An electrical stimulator forthe treatment of intractable pain syndromes, comprising: a pulsegenerator that generates digital signal pulses; a digital signalprocessor connected to said pulse generator that generates asine-wave-like output waveform that is further processed into first andsecond circuits; and two pairs of implantable electrodes connected tosaid digital signal processor and positioned on a subject's dorsalcolumn at predetermined locations to produce an interferential currentoutput waveform from said first and second circuits.
 24. The stimulatorof claim 23, wherein said interferential current output waveformincludes a base medium frequency of at least 500 Hz but no more than 20KHz.
 25. The stimulator of claim 23, wherein said interferential currentoutput waveform includes a resultant beat frequency of no more than 250Hz.
 26. An electrical stimulator for the treatment of intractable painsyndromes, comprising: a pulse generator that generates digital signalpulses; a field-programmable gate array connected to said pulsegenerator that generates a sine-wave-like output waveform that isfurther processed into first and second circuits; and two pairs ofimplantable electrodes connected to said field-programmable gate arrayand positioned on a subject's dorsal column at predetermined locationsto produce an interferential current output waveform from said first andsecond circuits.
 27. The stimulator of claim 26, wherein saidinterferential current output waveform includes a base medium frequencyof at least 500 Hz but no more than 20 KHz.
 28. The stimulator of claim26, wherein said interferential current waveform includes a resultantbeat frequency of no more than 250 Hz.
 29. A method for the treatment ofpersistent chronic pain syndromes using electrical stimulation of thespinal cord, said method comprising: connecting a pulse generator to adigital signal processor and supplying digital signal pulses to saiddigital signal processor which produces a sine-wave-like currentwaveform which is further processed and output to first and second pairsof implantable electrodes, wherein first and second circuits arecreated, respectively; positioning said first pair of implantableelectrodes on a subject's spinal cord at one set of diagonal corners ofa targeted area; positioning said second pair of implantable electrodeson the subject's spinal cord at the other set of diagonal corners of thetargeted area; and creating an interferential current with a base mediumfrequency of at least 500 Hz but no more than 20 KHz.
 30. The methodaccording to claim 29, wherein said method further includes varyinglocations of said first and second pairs of implantable electrodes alongthe spinal cord.
 31. The method according to claim 29, wherein saidmethod further includes modulating outputs of amplitudes of said firstand second circuits.
 32. The method according to claim 29, wherein saidmethod includes creating an interferential current with a resultant beatfrequency of no more than 250 Hz.
 33. The method according to claim 29,wherein said method includes creating the interferential current with avoltage output of 11 volts maximum for each circuit.
 34. The methodaccording to claim 29, wherein said method includes creating theinterferential current with a pulse width of 210 microseconds.
 35. Themethod according to claim 29, wherein said method includes creating theinterferential current with a pulse width comprising a range of at least10 microseconds but no more than 600 microseconds.
 36. The methodaccording to claim 29, wherein said method includes creating twointerferential currents by using two quadripolar leads.
 37. A method forelectrical stimulation of the spinal cord, said method comprising:connecting a pulse generator to a digital signal processor and supplyingdigital signal pulses to said field-programmable gate array whichproduces a sine-wave-like current waveform which is further processedand output to first and second pairs ofimplantable electrodes, whereinfirst and second circuits are created, respectively; positioning saidfirst pair of implantable electrodes on a subject's dorsal column at oneset of diagonal corners of a targeted area; positioning said second pairof surface electrodes on the subject's dorsal column at the other set ofdiagonal corners of the targeted area; and creating an interferentialcurrent with a base medium frequency of at least 500 Hz but no more than20 KHz.
 38. The method according to claim 37, wherein said methodfurther includes varying positions of said first and second pairs ofimplantable electrodes along the dorsal column.
 39. The method accordingto claim 37, wherein said method further includes modulating outputs ofamplitudes of said first and second circuits.
 40. The method accordingto claim 37, wherein said method includes creating an interferentialcurrent with a resultant beat frequency of no more than 250 Hz.
 41. Themethod according to claim 37, wherein said method includes creating aninterferential current with a pulse width of 210 microseconds.
 42. Themethod according to claim 37, wherein said method includes creating twointerferential currents by using two quadripolar leads.